The processing of material such as waste water residuals (sewage sludge), manure, yard waste, food processing wastes, etc., generally includes a stage in which the material is put into a form for subsequent use. Commonly, the material is pelletized for later use as fertilizer, for example.
Pellets are a desirable form because such wastes, even at the tail end of a processing stream have a large water component and relatively small pellets are more easily dried than non-pelletized material. In particular circumstances, such as the processing of sewage sludge into fertilizer, other materials are often mixed into the waste. Formation of the combined materials into evenly sized pellets results in a product in which the combined materials may be evenly distributed in use, such as during spreading onto a farmer's field. It has generally been found that it is necessary, or at least desirable to mix binding agents with the material of the processing stream prior to pelletization in order to ensure that the material have sufficient adhesion properties. Otherwise, the treated material might crumble apart, which is generally undesirable.
Several approaches to downstream pelletization of organic waste material and the like have been taken in the past. One pelletizer includes a large diameter disk having a shallow circumferential wall. The disk rotates about an axis perpendicular to the center of the disc and inclined to the horizontal. Moist material is fed onto the disk and sticks to the disk. As the disk rotates, pellets are formed.
Another approach involves a tilted cylindrical drum which rotates about the central axis of the drum. Material is fed into the raised end of the drum. Material is pelletized as the drum rotates. Interior drum walls having openings spaced radially inwardly of the drum periphery permit only the larger particles (which rise to the top of the rotating material) to flow towards the other end of the drum for eventual exit therefrom.
These two approaches produce the pellets desired in many situations, but suffer the disadvantage of being relatively slow. Pellet size and uniformity of size and shape of the pellets formed could be improved.
Yet another apparatus utilizes rollers which act against a cylindrical screen to force material through the screen web.
A known pelletizer utilizes a pair of horizontal rollers in abutting side-by-side contact with each other. The rollers rotate so that material may be fed downwardly into the crevice between the rollers. There are notches in each of the rollers which are aligned with each other so that material enters the notches, is compressed therein as the rollers rotate and expelled in pelletized form from the underside of the rollers.
In any event, to be effective, any pelletizer or pelletization process takes into account the fact that organic material being treated includes living matter, generally a bacterial component, the maintenance of which is generally desirable. For example, bacteria-containing sludge waste is desirable for use as fertilizer. Pelletization processes which kill or otherwise degrade the bacterial component to a degree sufficient to reduce the usefulness of the pelletized sludge as fertilizer are considered disadvantageous.